Headspace is the amount of air or open space you have in your container above the liquid. A common question when making cider is how much headspace should you have in your fermenter for primary fermentation? The next question is usually how much you should have when aging your hard cider. There are several considerations that can go into answering these questions but fundamentally, its about the aggressiveness of your fermentation and the risk of oxygen exposure. Let’s explore oxygen first.
Headspace & Oxygen
Oxygen is initially desired to support yeast reproduction or what we call fermentation. Yeast use oxygen to generate ATP, which is their energy source for cellular functions, like reproduction. Therefore, having a large amount of headspace in your primary fermentation vessel could be beneficial or at least not concerning. This added oxygen could be utilized by the yeast as they absorb nutrients and create ATP. The other key aspect to remember is the amount of CO2 yeast produce. As a reference, each specific gravity point (0.001) that your juice has will create around 0.6 volumes worth of CO2. If your juice has a gravity of 1.050 and you have 19 liters (5 gallons), you will create about 570 liters (150 gallons) of CO2. Whether you have 1 liter of headspace or 10 liters, you won’t really need to worry too about headspace for your primary fermentation. The biggest risk is if your fermentation process is delayed or slow, which could be caused by temperature, sulfur dioxide (SO2), acid (low pH), or the use of natural yeast. In a slow start, all that oxygen may enable other organisms, like acetic acid bacterias and mold to grow. Many of the things slowing your fermentation also slow bacteria and mold growth as well. That isn’t the same when we are considering the aging or secondary fermentation of hard cider.
Once all the sugar is consume by yeast, they pack up and go home. Well, they absorb everything they can and begin precipitating out by flocculating together and falling to the bottom of the fermenter. Small amounts remain but the yeast that turned your juice into cider takes a back seat to other organisms. The most common of these are Brettanomyces yeasts, lactic acid bacteria (LAB), acetic acid bacteria, and flor or film yeasts. Some of these organisms, like LAB and some Brett strains, don’t use oxygen. Their processes are anaerobic. Malolactic fermentation (MLF) is a great example of an anaerobic process performed by LAB. Others organisms, like acetic acid bacteria and film yeast, require oxygen. If you rack your cider into a new container, open your fermenter to draw out a sample, or even just leave it under and airlock, having a large headspace means the potential for large amounts of oxygen. At this stage, oxygen is only enabling organisms that generally don’t improve your hard cider. Oxygen can also enable oxidation reactions with various compounds, like phenolic compounds. The recommended option is to minimize headspace for secondary storage and aging of cider. Otherwise, you risk formation of film yeasts, acetic acid, and oxidation of some compounds.
Headspace & Fermentation Rate
The aggressiveness of your fermentation depends on your yeast, the juice, and your temperature. That rate or aggressiveness can make you wish your headspace was bigger. It is common for me to add just a little more juice to my fermenter because I’m trying to avoid needing another container. My recommendation is to stop, put it in a glass and enjoy fresh juice as a reward for your hard work pressing apples or freeze it for topping your cider for storage. Having sufficient headspace is only fully appreciated when your fermenter is shooting yeast and cider on your walls or cabinet ceiling! Trust me, I have been there. The other option is to control the fermentation rate. Pectin and solids in your juice will increase the need for more headspace. Pectin will rise to the top as CO2 is released and help form the yeast cap that can quickly fill that ample amount of headspace you thought you had. Adding pectic enzyme to juice, allowing the pectin to settle out, racking off the clear juice from this sediment, and fermenting this clearer juice will reduce the fermentation aggressiveness and the formation of the yeast cap. This allows you to need less headspace. Cooler fermentations will slow the fermentation rate and the risk of your headspace filling with a massive yeast cap that fouls your airlock along with everything around it. The answer again for primary fermentation where you risk aggressive fermentations is to have a reasonably large headspace. For me, that would means 10% of your liquid volume but maybe even more. The only time I don’t recommend ample headspace is when you are doing a secondary ferment. Even when adding an adjunct that has some sugar or allowing a fermentation to finish slowly after an initial rack, the fermentation rate will generally be slow and I would recommend little to no headspace in your container.
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